Vapor recovery fuel dispensers, particularly gasoline dispensers, have been known for quite some time, and have been mandatory in California for a number of years. The primary purpose of using a vapor recovery fuel dispenser is to retrieve or recover the vapors, which would otherwise be emitted to the atmosphere during a fueling operation, particularly for motor vehicles. The vapors of concern are generally those which are contained in the vehicle gas tank. As liquid gasoline is pumped into the tank, the vapor is displaced and forced out through the filler pipe. Other volatile liquids such as hydrocarbon fluids raise similar issues.
A traditional vapor recovery apparatus is known as the "balance" system, in which a sheath or boot encircles the liquid fueling spout and connects by tubing back to the fuel reservoir. As the liquid enters the tank, the vapor is forced into the sheath and back toward the fuel reservoir or underground storage tank (UST) where the vapors can be stored or recondensed. Balance systems have numerous drawbacks, including cumbersomeness, difficulty of use, ineffectiveness when seals are poorly made, and slow fueling rates.
As a dramatic step to improve on the balance systems, Gilbarco, Inc., assignee of the present invention, patented an improved vapor recovery system for fuel dispensers, as seen in U.S. Pat. No. 5,040,577 to Pope, which is herein incorporated by reference. The Pope patent discloses a vapor recovery apparatus in which a vapor pump is introduced in the vapor return line and is driven by a variable speed motor. The liquid flow line includes a pulser, conventionally used for generating pulses indicative of the liquid fuel being pumped. This permits computation of the total sale and the display of the volume of liquid and the cost in a conventional display, such as, for example as shown in U.S. Pat. No. 4,122,524 to McCrory et al. A microprocessor translates the pulses indicative of the liquid flow rate into a desired vapor pump operating rate. The effect was to permit the vapor to be pumped at a rate correlated with the liquid flow rate so that, as liquid is pumped faster, vapor is also pumped faster.
There are three basic embodiments used to control vapor flow during fueling operations. The first embodiment is the use of a constant speed vapor pump during fueling without any sort of control mechanism. The second is the use of a pump driven by a constant speed motor coupled with a controllable valve to extract vapor from the vehicle gas tank. While the speed of the pump is constant, the valve may be adjusted to increase or decrease the flow of vapor. The third is the use of a variable speed motor and pump as described in the Pope patent, which is used without a controllable valve assembly. All three techniques have advantages either in terms of cost or effectiveness, and depending on the reasons driving the installation, any of the three may be appropriate. The present state of the art is well shown in commonly owned U.S. Pat. No. 5,345,979, which is herein incorporated by reference.
Regardless of whether the pump is driven by a constant speed motor or a variable speed motor, there is no feedback mechanism to guarantee that the amount of vapor being returned to the UST is correct. A feedback mechanism is helpful to control the A/L ratio. The A/L ratio is the amount of vapor-air being returned to the UST divided by the amount of liquid being dispensed. An A/L ratio of 1 would mean that there was a perfect exchange. Often, systems have an A/L &gt;1 to ensure that excess air is recovered rather than allowing some vapor to escape. This inflated A/L ratio causes excess air to be pumped into the UST, which results in a pressure build up therein. This pressure build up can be hazardous, and as a result most USTs have a vent that releases vapor-air mixtures resident in the UST to the atmosphere should the pressure within the UST exceed a predetermined threshold. While effective to relieve the pressure, it does allow hydrocarbons or other volatile vapors to escape into the atmosphere.
While PCT application Ser. No. PCT/GB98/00172 published Jul. 23, 1998 as WO 98/31628, discloses one method to create such a feedback loop using a Fleisch tube, there remains a need to create alternate feedback mechanisms to more accurately measure the vapor flow in a vapor recovery system in order to minimize the need to vent the UST to the atmosphere and ensure proper vapor recovery.